ComboBox choice_values full of empty strings despite PDF having valid choices.

[sarahkittyy]

Description of the bug

I am using the 940b: https://www.irs.gov/pub/irs-pdf/f940b.pdf

The PDF file has identical pages, and each page has this specific dropdown:

The choice_values variable is empty.

import pymupdf

pdf = pymupdf.open('f940b.pdf')

for page in pdf:
    for widget in page.widgets():
        if widget.field_type_string == 'ComboBox':
            print(widget.choice_values)
        widget.update()
pdf.save('f940b-output.pdf')

Expected output:

[' - Select One - ', '  ', 'Cincinnati, OH 45999', 'Memphis, TN 37501', 'Ogden, UT 84201', 'Philadelphia, PA 19255']
[' - Select One - ', '  ', 'Cincinnati, OH 45999', 'Memphis, TN 37501', 'Ogden, UT 84201', 'Philadelphia, PA 19255']

Actual output:

['', '', '', '', '', '']
[' - Select One - ', '  ', 'Cincinnati, OH 45999', 'Memphis, TN 37501', 'Ogden, UT 84201', 'Philadelphia, PA 19255']

This also affects the resulting f940b-output.pdf, where the first combo box is suddenly completely empty with no choices available.

How to reproduce the bug

See above

PyMuPDF version

1.24.13

Operating system

Linux

Python version

3.12